1. Field of the Invention
The present invention relates to an optical transmission module configured to output an optical signal of light emitted from a light emitting device such as a semiconductor laser or the like, and an optical transmitter using it.
2. Related Background Art
In optical transmission systems using light as information transmission media, an optical transmission module provided with a semiconductor light emitting device such as a semiconductor laser is used as a transmitting means for transmitting an optical signal to be transmitted through an optical transmission line such as an optical fiber transmission line.
FIG. 11 is a front sectional view showing a configuration of a conventional optical transmission module. This optical transmission module 9 is constructed in the configuration wherein semiconductor laser 92 is mounted on pole 91 provided nearly perpendicularly to stem 90. Ahead of the semiconductor laser 92 in the opposite direction to the stem 90, condenser lens 93 and optical fiber 94 are located in order to output the light emitted from the semiconductor laser 92, as an optical signal to the outside.
A surface reception type monitor photodiode 96 is placed through submount 95 on the stem 90. This monitor photodiode 96 is a device that detects light emitted backward from the semiconductor laser 92 to monitor the operating condition of semiconductor laser 92 and that is located on the axis of emission of the backward light from the semiconductor laser 92, or at a position near it (e.g., reference is made to Document 1 “Japanese Patent Applications Laid-Open No. 59-96789”, Document 2 “Japanese Patent Applications Laid-Open No. 61-156784”, and Document 3 “Japanese Patent Applications Laid-Open No. 63-200590”).
In the optical transmission module 9 using the semiconductor laser 92, the intensity of emitted light generally varies with change in ambient temperature, because of temperature dependence of emission output of the semiconductor laser 92. As a solution thereto, the photodiode 96 is used to monitor the operating condition of the semiconductor laser 92, and the drive current supplied to the semiconductor laser 92 is subjected to such feedback control as to keep constant the value of current monitored by the photodiode 96. This driving method of semiconductor laser is called APC (Automatic Power Control) driving and is adopted in the optical transmission modules using the light emitting devices such as semiconductor lasers or the like.